Fluorine-containing surfactants represented by the general formula RfCOOM (Rf: a perfluoroalkyl group, and M: an alkali metal, an ammonium group, an organic amine salt or a hydrogen atom) have been so far widely used in the emulsion polymerization of fluorine-containing monomers. Perfluorooctanoic acid (salts) C7F15COOM, which is one of them and a typical example, is well known as the most distinguished surfactant, because of its good monomer emulsificability and latex stability, and easy washability following the salting-out operation.
However, it is reported that compounds containing a perfluoroalkyl group having 7 or more carbon atoms are biologically degraded in the environment and converted to those having relatively high bioaccumulation and environmental condensation, causing concerns for exposure during treatment processes, and for release or diffusion from waste, treated substrates, etc., into the environment.
In contrast, compounds containing a perfluoroalkyl group having 6 or less carbon atoms have low persistence in the environment or the human body; however, their critical micelle concentration [CMC] is high, and desired sufficient emulsification performance cannot be thus obtained.
A possible means for imparting environmental degradability is that a hydrogenated part is formed in the perfluorinated hydrophobic group in a fluorine-containing surfactant compound. Further, as a means for weakening the environmental persistence of decomposed products, it is considered desirable that the hydrophobic group has 6 or less of continuous perfluorinated-carbon atoms, such as RfCnH2nCmF2m-(Rf: a C1-C6 perfluoroalkyl group, n: an integer of 1 or more and m: an integer of 1 to 6).
The present applicant has previously reported that a polyfluoroalkane carboxylic acids (salts) represented by the general formula CnF2n+1(CH2CF2)m(CF2CF2)1-1CF2COOM (M: H, an NH4 group or an alkali metal, n: 1 to 6, m: 1 to 4 and 1:1 or 2) effectively serves as a surfactant having excellent monomer emulsifiability and latex stability when used as an emulsifying agent or a dispersing agent in the polymerization reaction of fluorine-containing monomers, and that the polyfluoroalkane carboxylic acids (salts) effectively serves as a surfactant that can increase the micellar dissolution of fluorine-containing monomers, such as vinylidene fluoride, when used as an emulsifying agent or a dispersing agent in the homopolymerization or copolymerization reaction of vinylidene fluoride (Patent Document 1).
Patent Document 1 indicates that the amount of vinylidene fluoride dissolved in an aqueous polyfluoroalkane carboxylic acid ammonium solution is larger than the amount of vinylidene fluoride dissolved in an aqueous perfluorooctanoic acid ammonium solution at the same temperature and the same pressure; however, the critical micelle concentration and the surface tension at that time are nowhere described.